The invention relates to a circuit arrangement for operating a discharge lamp with a high frequency current comprising
input terminals for connection to a source of low frequency supply voltage,
rectifier means coupled to said input terminals for rectifying said low frequency supply voltage,
a first circuit comprising a series arrangement of first unidirectional means, second unidirectional means and first capacitive means coupled to a first output terminal N3 of said rectifier means and a second output terminal N5 of said rectifier means,
inverter means shunting said first capacitive means for generating the high frequency current,
a load circuit comprising a series arrangement of inductive means, second capacitive means and means for applying a voltage to the discharge lamp, said load circuit connecting a terminal N1 of said inverter means to a terminal N2 between the first unidirectional means and the second unidirectional means, and
a second circuit comprising third capacitive means and connecting terminal N2 to terminal N5,
a third circuit connecting the first output terminal N3 of the rectifier means to a terminal N4 between the second unidirectional means and the first capacitive means, said third circuit comprising a series arrangement of third unidirectional means and fourth unidirectional means, and
a fourth circuit connecting a terminal N7 between said third unidirectional means and said fourth unidirectional means to a terminal N6 that is part of the load circuit.
Such a circuit arrangement is known from WO /19578. In the known circuit arrangement the discharge lamp is simply mounted in series with the inductive means and second capacitive means comprised in the load circuit. During lamp operation the known circuit arrangement supplies a high frequency current to the lamp that is generated by the inverter. The known circuit arrangement comprises two power feedback loops. The first power feedback loop is formed by the load circuit and the first and second unidirectional means. The second power feedback loop is formed by the fourth circuit and the third and fourth unidirectional means. Due to these two power feedback loops, the known circuit arrangement generates only a small amount of THD during operation while it comprises only a relatively small amount of relatively cheap components. It has been found that the known circuit arrangement is very suitable for supplying lamps that during operation have a peak to peak lamp voltage that is smaller than the peak value of the low frequency supply voltage. In case the peak to peak voltage of the discharge lamp is increased to a value higher than the peak value of the low frequency supply voltage, THD increases and the operational conditions begin to deviate from optimal conditions.
The operating point could be restored by incorporating a transformer equipped with primary winding and a secondary winding into the means for applying a voltage to the discharge lamp. The primary winding would have to be part of the load circuit and would preferably have to be dimensioned so that the peak to peak voltage over it is roughly equal to the peak value of the low frequency supply voltage. At the same time the secondary winding would have to be dimensioned as is necessary to generate the lamp voltage. An important disadvantage of this solution, however, is that the current in the load circuit and in the switches incorporated in the inverter becomes relatively high. In practice this means that either expensive components that are capable of carrying this relatively high current need to be used or that the components need to be cooled during operation.
Alternatively, operating point can also be restored by incorporating a voltage dividing network in the second power feedback loop. It was found, however, that the presence of such a voltage dividing network in some cases gave rise to a relatively high current in the load circuit and in the switches of the inverter. This high current is disadvantageous for the reasons outlined hereabove. In other cases it was found that the presence of a voltage dividing network made it necessary to operate the inverter at a frequency that was relatively close to the resonance frequency of the load circuit. This way of operating the inverter causes a high tolerance sensitivity of the components of the load circuit and can cause a relatively high power dissipation in the switches of the inverter.
It is an object of the present invention to provide a circuit arrangement, that causes relatively little harmonic distortion of the low frequency supply current, while the circuit arrangement is also capable of operating discharge lamps having a relatively high lamp voltage without the drawbacks of components comprised in the load circuit and the inverter having to conduct a relatively large current during lamp operation or a high tolerance sensitivity of the components of the load circuit or a relatively high power dissipation in the switches of the inverter.
A circuit arrangement according to the invention is for this purpose characterized in that the means for supplying a voltage to the discharge lamp comprise a transformer equipped with a series arrangement of two primary windings comprised in the load circuit and a secondary winding and in that terminal N6 is situated between the two primary windings.
It has been found that a circuit arrangement according to the invention can be dimensioned for a very wide range of lamp voltages in such a way that the current in the load circuit and the inverter switches is relatively low while the tolerance sensitivity of the components of the load circuit is quite acceptable.
It has also been found that a smooth operation of the circuit arrangement could be realized in case the second circuit further comprises the first capacitive means.
A smooth operation of the circuit arrangement was also found for configurations of the circuit arrangement wherein the fourth circuit comprises fourth capacitive means.
The unidirectional means preferably comprise diode means. The unidirectional means are thus realized in a very simple way.
In a preferred embodiment of a circuit arrangement according to the invention the inverter means comprise a series arrangement of a first switching element, terminal N1 and a second switching element, and a drive circuit DC coupled to the switching elements for generating a drive signal for rendering the switching elements alternately conducting and non-conducting. The inverter is thus realized in a relatively simple and dependable way.
Preferably the transformer is constructed in such a way that the first primary winding L3 and the second primary winding L4 are formed by one physical winding and terminal N6 is formed as a tap on the winding. In that way the transformer is constructed in a relatively simple and compact way.